|Impact of Rice Straw
ation on Yield Potential-01
Project Leader and Principal UC Investigators
William R. Horwath, professor, Department of Land, Air and Water Resources, University of California, Davis
|Post-harvest management of rice straw is one of the critical
issues facing California rice farmers. The central question guiding this project during
2001 is whether factors other than nitrogen fertility could be adversely affecting maximum
yield potential in fields managed with straw incorporation and how this compares with
burning. Objectives were:
California growers have spent more than a decade reinventing some aspects of rice farming since passage of the California Rice Straw Burning Reduction Act in 1991. This legislation forced growers to contend with a tremendous amount of rice residue that used to be burned. Researchers have learned much since this project got under way in 1993.
The historic practice of burning rice residue served many functions, including reducing the incidence of weeds and pathogens and to facilitate seedbed preparation. Burning also enhanced fertility, most likely by increasing the availability of phosphorous, potassium and micronutrients. Now,growers must either haul it away or find an agronomically sound way of dealing with straw in the field. Incorporation into the soil and rolling it onto the soil surface are currently two of the most common practices.
These in-field approaches were thought to immobilize nutrients and increase the incidence of pests, leading to possible yield declines. Long-term research has shown, however, that winter flooding increases the availability of soil nitrogen and leads to a significant yield gain over unflooded fieldsregardless of residue management. This response is more pronounced on soils with higher clay or organic matter content. Additional yield gain is seen in fields with a minimum of three to five years of straw incorporation (compared to burning or removing).
These findings suggest that fertilizer rates can be reduced by up to 50 pounds/acre, depending on soil type, residue management and the use of winter flooding. The fertility status of other nutrients under in-field residue management has not been as closely examined.
The in-field straw management approach leads to soil organic matter build-up, a positive soil characteristic. However, this increased organic matter also leads to a larger microbial biomass and immobilization of phosphorous and micronutrient metals in soil organic matter. The positive response to chicken manure application, as seen by some growers, suggests that phosphorous and possibly micronutrients are beginning to become impacted by soil organic matter accumulation. For these reasons, a comprehensive fertility guide for rice growers in California must include all macro- and micronutrients.
Nitrogen status is fairly complex. Fertilizer nitrogen additions increase yields on straw-incorporated treatments up to 100 pounds N/acre, while the burned treatment yields continue to rise beyond that amount. This suggests that straw incorporation could be yield-limiting, possibly from weeds, disease or pathogen pressure. The long-term impact of straw incorporation on weeds and pathogens is uncertain. What is certain is that herbicide use will become increasingly important in straw-incorporated management approaches where weed seeds are protected from waterfowl foraging. In treatments such as cage rolling, waterfowl foraging can significantly reduce weed incidence.
Incorporation of rice will likely remain a major practice for residue management in California. A decline in soil potassium has been observed at all research sites where rice straw is removed. Thus, continual straw removal could lead to a long-term decline in potassium fertility.
Maximum Yield Potential
An experiment continued to determine the maximum yield potential of rice following prolonged straw incorporation. After eight years of studying alternative rice straw management, grain yields did not decline compared to burned fields. As previously noted, the threshold beyond which no further yield increases were observed under a straw-incorporated field is 100 pounds/acre. Thus, in fields relying on straw incorporation nitrogen fertilizer applications can be limited to this amount. A similar but smaller effect would be expected in fields using rolling and other surface residue practices.
The possible increase in weed and disease pressure in straw-incorporated or rolled fields was not severe enough to cause a grain-yield decline under current fertilizer practices. However, considerable attention to weeds and herbicide application was required to attain these results. This represents a potential cost increase for managing the residue and herbicide application.
An increase in soil nitrogen availability could lead to an increase in weed and disease pressure. Based on ongoing research at other sites, it is likely that factors other than nitrogen and potassium are involved in controlling maximum yield potential. On the west side of the Sacramento Valley, indications are that magnesium, calcium and phosphorous levels are also related to yield.
Weed and Disease Pressure
Changes in agronomic practices that impact crop yield may take a decade or longer to manifest. During the transition, the direction of that change is difficult to predict. Incorporated and rolled plots at the long-term study site near Maxwell showed a tendency for both higher weed and disease symptoms. Although herbicides effectively controlled most weeds at this location, watergrass continues to be a problem.
A number of deleterious rhizobacteria (root pathogens) have been isolated from both rice roots and weed roots. These bacteria normally stop root growth. Some inhibit shoot growth as well. Burned plots show the lowest level of pathogenic bacteria, while the incorporated, winter-unflooded plots showed the highest number. In addition, the incorporated plots had the highest number of seed-killing pathogens. Research conducted in 2001 did not assess whether these organisms affected rice growth but will be given a closer look in 2002with particular attention paid to yield impacts.